The present invention relates to an image forming output apparatus such as a digital color copying apparatus, a digital color printer, and the like to which an electrophotographic process is applied.
In a copying apparatus or printer to which an electrophotographic process is applied, an image is formed in the following manner: a rotating cylindrical photoreceptor or a belt-shaped photoreceptor is rotated and electrostatic latent images are formed thereon successively; black toner and other color toners, in the case of color image formation, are adhered to the electrostatic latent images formed as described above for development; and they are transferred onto a recording sheet, thus, the image is obtained. In this specification, the photoreceptor drum in the image output apparatus and a driving roller for the belt-shaped photoreceptor are referred to as a rotational body. When the rotational speed of the photoreceptor drum is varied for some reasons, jittering or an uneven image is caused in the outputted image. These phenomena have remarkably appeared especially in the digital system electrophotographic technology employing scanning by means of a semiconductor laser for writing images on a photoreceptor. Fluctuation in rotational speed of the photoreceptor has caused speed fluctuation of a writing system in the subsidiary scanning direction to create a slight variation in the distance between writing lines, contributing to the remarkable deteriorations of the image quality.
Conventionally, when designing the driving system for use in a copying apparatus or a printer, the main consideration is that the objects driven by the driving device are appropriately located in the allowable space, while satisfying the values of the line speed or number of revolutions introduced from the product specification. That is, the following are main concerns: the method by which the driving power is transmitted from a driving power source to a driven object; and mechanical elements for power transmission. Accordingly, when jittering and rotational fluctuation are caused in the product, the cause is investigated, and one or more of the following countermeasures are considered: a bearing of a drive shaft of the photoreceptor is replaced with one made of sintered metal; a flywheel is connected with the drive shaft of the photoreceptor; a brake, in which a spring is combined with a friction material, is provided on the rotary shaft of the photoreceptor drum; the accuracy of a gear is enhanced; or a helical gear with various kinds of torsion angles is provided.
However, in the development of a digital type image output apparatus, strict reproducibility of a one dot line written by a laser beam is required with an improvement of the apparatus performance, and accuracy required on the driving system has rapidly become strict. The accuracy required is a level at which the uniformity of laser writing in the subsidiary scanning direction is assured in relation to the visible sensitivity of the visual system. In order to accomplish this accuracy, it is mostly necessary to make the photoreceptor driving system highly accurate. The main factor of the rotational fluctuation of the driving system is the following: the rotational fluctuation per one rotation of the rotating shaft of a motor is large, and absolute values of fluctuation components per one rotation of a gear and per one tooth of a gear are large; and fluctuation components and their higher harmonic wave components cause a resonance phenomenon in relation to the proper oscillation frequency of the driving system.
FIG. 10 shows the power spectrum of speed fluctuation of conventional apparatuses. In FIG. 10, fluctuation components of a gear according to the line speed proper to the apparatus are 176 Hz in the case of a gear directly coupled to the motor, 64 Hz in the case of a second shaft, and 25 Hz in the case of a gear directly coupled to a drum, and in this case, a higher harmonic wave component of 50 Hz is shown. Further, a component of a rotation of the gear directly coupled to the motor is 22 Hz, and its higher harmonic wave component of 44 Hz is shown in the drawing.
In FIG. 11, an example is shown in which the transfer function has been measured in order to numerically obtain the proper oscillation frequency of the driving system. In this case, the measurement has been conducted in the following way: an output of an impact excitation hummer, and an output of a piezoelectric type pick-up sensor, provided to one end of a photoreceptor drum in order to measure the fluctuation of the acceleration in the rotation direction, are connected with a dual channel type FFT analyzer; and a Fourier spectrum ratio is obtained. From FIG. 11, the following can be found: a peak of the proper oscillation frequency is near 45 Hz; and high level areas of the transfer function are spread near the range of 30 to 60 Hz.
FIG. 12 shows superimposition of the fluctuation component spectrum and the transfer function. In the driving system, it can be found from the drawing that a peak of the transfer function and the position of a frequency area, to which the fluctuation component and its second harmonics belong, are superimposed. That is, it is found that the driving system amplifies the fluctuation components (resonance is caused).
Actually, when data measured from three apparatuses having the different driving systems were investigated, the fluctuation of rotation of the photoreceptor was 5 to 8%.